With the help of the teacher, I improved my ____ (speak/speaking) in English.
下列关于隋朝大运河的叙述,错误的是( )
数列an的前n项和为Sn,已知a1=1,a2=12,an+2=an+1-an,则S2013的值为 ( )
因式分解:x2﹣49=____.